[4480] in SIPB bug reports
Re: Mosaic
daemon@ATHENA.MIT.EDU (Bill Sommerfeld)
Thu Mar 17 12:25:58 1994
Date: Thu, 17 Mar 1994 12:23:00 -0500
From: Bill Sommerfeld <sommerfeld@apollo.hp.com>
To: yandros@MIT.EDU
Cc: bert@MIT.EDU, bug-sipb@MIT.EDU
In-Reply-To: <9403170310.AA23541@infocalypse.MIT.EDU> (yandros@MIT.EDU)
From: yandros@MIT.EDU
Date: Wed, 16 Mar 94 22:10:44 EST
Cc: bert@MIT.EDU, bug-sipb@MIT.EDU
X-Orgs: MIT SIPB VWA DCNS
X-Shabby: not
No, it just means that the http designers screwed up when they didn't
include any provisions for caching or cache coherance in http.
EXcuse me? I'm not sure where this comes from. The problem Eric was
having was that the image was cached. What are you saying?
A "cache" is a small, fast memory put in front of a large, slow
memory, which provides the illusion of the large, slow memory being a
large, fast memory.
If the "large, slow" memory is shared between multiple users, there
should be some way for changes to an object made by one user to
automatically replace copies of that object in caches of another user.
This is what "cache coherance" means.
AFS includes cache coherance hints in its file system protocol; the
file server keeps track of which users have cached copies of an object
and informs them to invalidate their caches if someone updates a file.
The internet DNS includes explicit "time to live" information with
returned information; once the TTL expires, the cache is flushed.
Sites making frequent changes set the TTL to a small value; sites
making infrequent changes set it to a large value. This is not as
strict as the AFS model, but requires no changing state on the server.
HTTP does not include *any* support for cache coherance -- either
callbacks or TTL -- and this is a major shortcoming of the protocol,
which prevents it from *really* scaling up in the same way that AFS or
DNS scale up; any "TTL" that an HTTP client assumes is going to be
wrong in at least some environments, and an infinite ttl (which is
what Mosaic uses for inlined images) is Just Plain Wrong.
- Bill
